Similar to a regular fail, a cool-fail is when someone attempts to do something they do at a regular time to show off (Eg keeps missing 3 point shots in a basketball game even though they play basketball) but continuosly fails.

"Wow, he's taller than evryone here, he could have scored at least 10 points without trying to score 3 pointers to show off. He missed all of them. Cool-fail.